1. Field of the Invention
The present invention relates to a light-emitting element driving device for driving a light-emitting element such as a light-emitting diode (LED) or the like which emits light at a luminance level depending on a current flowing therethrough, and a display device having a non-emission transmissive display unit which incorporates such a light-emitting element driving device.
2. Description of the Related Art
The backlight of a liquid crystal display panel employs LEDs as its light source which have replaced a CCFL (Cold Cathode Fluorescent Lamp) employing a fluorescent tube.
Particularly, backlights including individual primary LEDs such as red LEDs, green LEDs, and blue LEDs for producing white light according to an optical additive color synthesis have been used in television applications as they can easily achieve good color balances. In recent years, white LEDs with improved color rendition have widely been used television applications.
An LED basically has such characteristics that its luminance varies depending on a current supplied thereto, and has a forward voltage which differs depending on individual LED variations and temperatures.
Therefore, when LEDs are used as the backlight of a liquid crystal display panel, a driving device for those LEDs is required to have constant-current characteristics in order to achieve a constant uniform luminance level.
There is known a driving device which adopts a PWM control process for turning on and off a current flowing through an LED with certain timing and adjusting a luminance level based on the ratio of the on and off periods in order to adjust the luminance level stably in a wide dynamic range.
According to one of schemes for realizing the PWM control process, a switch element is inserted in series to the LED to turn on and off the LED with prescribed timing (see Japanese Patent Laid-Open No. 2001-272938), for example).
There is also known a process for turning on and off switch elements connected in series to LEDs with energization signals to control switching transistors of a switching power supply such as a boosting chopper or the like according to a PWM control process.
FIG. 1 of the accompanying drawings is a circuit diagram, partly in block form, of a light-emitting element (LED) driving device according to the related art.
As shown in FIG. 1, an LED driving device 1 includes a booster-chopper-type switching power supply 2 and a plurality of light emitters 3-1 through 3-n (n=2 in FIG. 1) as loads including LED arrays each including a plurality of series-connected LEDs. It is assumed that n=2 in the description which follows.
The LED driving device 1 also includes a constant-current controlling switching transistor 4-1 and a constant-current circuit 5-1 which are connected in series to the light emitter 3-1, and a constant-current controlling switching transistor 4-2 and a constant-current circuit 5-2 which are connected in series to the light emitter 3-2.
The LED driving device 1 further includes switch drivers 6-1, 6-2, a minimum voltage selecting circuit 7, and a control circuit 8.
The switching power supply 2 includes a constant-voltage source V21, an inductor L21, a diode D21, an electric storage capacitor C21, a switching transistor SW21, a current detecting resistive element R21, and nodes ND21 through ND23.
The inductor L21 has an end connected to the constant-voltage source V21 which has a voltage VDD and an opposite end connected to the node ND21. The diode D21 has an anode connected to the node ND21 and a cathode connected to the node ND22. The capacitor C21 has a terminal (electrode) connected to the node ND22 and another terminal (electrode) connected to a reference potential VSS, e.g., a ground potential.
The node ND22 is connected as a voltage output node of the switching power supply 2 to respective ends of the light emitters 3-1, 3-2.
The switching transistor SW21 includes an NMOS transistor which is an n-channel field-effect transistor, for example. The switching transistor SW21 has a drain connected to the node ND21 and a source connected to an end of the resistive element R21. The other end of the resistive element R21 is connected to the reference potential VSS.
The switching power supply 2 thus constructed operates as follows: The control circuit 8 supplies a PWM-controlled pulse signal to turn on and off the switching transistor SW21 to boost the voltage VDD of the constant-voltage source V21. The switching power supply 2 supplies the boosted voltage VDD as a voltage Vo to the ends of the light emitters 3-1, 3-2.
Each of the light emitters 3-1, 3-2 includes a series-connected array of LEDs 31 through 3m. 
The LED 31 on an end of the series-connected array of each of the light emitters 3-1, 3-2 has an anode connected to the voltage output node ND22 of the switching power supply 2.
The LED 3m on the other end of the series-connected array of the light emitter 3-1 has a cathode connected to the drain (one terminal) of the switching transistor 4-1.
The LED 3m on the other end of the series-connected array of the light emitter 3-2 has a cathode connected to the drain (one terminal) of the switching transistor 4-2.
Each of the light emitters 3-1, 3-2 is not limited to a plurality of LEDs, but may include a single LED.
The switching transistor 4-1 has a source (other terminal) connected to a terminal of the constant-current circuit 5-1, whose other terminal is connected to the reference potential VSS.
The switching transistor 4-1 remains turned on during the period of an active high level of a pulsed LED energization signal LO1 that is supplied via the switch driver 6-1 to the gate of the switching transistor 4-1.
At this time, a current ILED flows into the light emitter 3-1 which is supplied with the voltage Vo from the switching power supply 2, energizing the LEDs 31 through 3m of the light emitter 3-1.
The switching transistor 4-1 remains turned off during the period of a non-active low level of the pulsed LED energization signal LO. At this time, no current ILED flows into the light emitter 3-1 which is supplied with the voltage Vo from the switching power supply 2, de-energizing the LEDs 31 through 3m of the light emitter 3-1.
While the switching transistor 4-1 is being energized, a monitor voltage Vs1 at a junction node ND1 between the switching transistor 4-1 and the constant-current circuit 5-1 is as follows:
The monitor voltage Vs1 is calculated by subtracting the sum ΣVf (=VF) of forward voltages Vf of all the LEDs 31 through 3m of the light emitter 3-1 from the voltage Vo supplied from the switching power supply 2.
The monitor voltage Vs1 thus calculated does not take into account a voltage drop across the switching transistor 4-1.
If the switching transistor 4-1 includes a field-effect transistor (FET), then the monitor voltage Vs1 at the junction node ND1 is calculated by subtracting the sum ΣVf of the forward voltages Vf of all the LEDs 31 through 3m of the light emitter 3-1 and a drain-to-source voltage Vds1 of the FET as the switching transistor 4-1 from the voltage Vo supplied from the switching power supply 2.
The constant-current controlling switching transistor 4-2 has a source (other terminal) connected to a terminal of the constant-current circuit 5-2, whose other terminal is connected to the reference potential VSS.
The switching transistor 4-2 remains turned on during the period of an active high level of a pulsed LED energization signal LO2 that is supplied via the switch driver 6-2 to the gate of the switching transistor 4-2.
At this time, a current ILED flows into the light emitter 3-2 which is supplied with the voltage Vo from the switching power supply 2, energizing the LEDs 31 through 3m of the light emitter 3-2.
The switching transistor 4-2 remains turned off during the period of a non-active low level of the pulsed LED energization signal LO2. At this time, no current ILED flows into the light emitter 3-2 which is supplied with the voltage Vo from the switching power supply 2, de-energizing the LEDs 31 through 3m of the light emitter 3-2.
While the switching transistor 4-2 is being energized, a monitor voltage Vs2 at a junction node ND2 between the switching transistor 4-2 and the constant-current circuit 5-2 is as follows:
The monitor voltage Vs2 is calculated by subtracting the sum ΣVf (=VF) of forward voltages Vf of all the LEDs 31 through 3m of the light emitter 3-2 from the voltage Vo supplied from the switching power supply 2.
The monitor voltage Vs2 thus calculated does not take into account a voltage drop across the switching transistor 4-2.
If the switching transistor 4-2 includes a field-effect transistor (FET), then the monitor voltage Vs2 at the junction node ND2 is calculated by subtracting the sum ΣVf of the forward voltages Vf of all the LEDs 31 through 3m of the light emitter 3-2 and a drain-to-source voltage Vds2 of the FET as the switching transistor 4-2 from the voltage Vo supplied from the switching power supply 2.
The minimum voltage selecting circuit 7 selects a minimum voltage Vsmin from the monitor voltages Vs1, Vs2 at the nodes ND1, ND2 which are calculated by subtracting the voltage drops across the light emitters 3-1, 3-2 and the switching transistors 4-1, 4-2 from the voltage Vo, and supplies the selected minimum voltage Vsmin to the control circuit 8.
The control circuit 8 supplies the gate of the switching transistor SW21 with a pulse signal having a pulse duration depending on the minimum voltage Vsmin selected by the minimum voltage selecting circuit 7.
The switching power supply 2 boosts the voltage VDD of the constant-voltage source V21 by turning on and off the switching transistor SW21 with the pulse signal supplied to the gate thereof.
In this manner, the voltage at the constant-current control terminal of the light emitter 3-1 or 3-2 under the maximum voltage VF is controlled at a constant level.